POLYPI. 183 



Hall. The smaller specimens are individuals, probably of the same species, 

 which have not their full development, nor even, (save in the one which still 

 lacks a central disc) the fall number of arms. The two smallest figures repre- 

 sent the germs of two distinct species of Graptolites. 



Size, 7 x6 . Price, $0.G0. 



CLASS III.— POLYPI. 



These soft bodied, aquatic Zoophytes, represented by the fresh-water 

 Hydras and the Sea- Anemones and Corals of the ocean, have cylindrical, 

 oval and oblong bodies, with an aperture at one end which is surrounded 

 by a coronet of tentacles. From the inner surface project radiating 

 partitions, leaving an open space in the centre. The Corals have the 

 power of secreting a corrallum ; and this secretion is as truly internal 

 as the bones of the Vertebrates. It exactly copies the animal : the 

 rays correspond to the partitions, and the tubular cavity to the spac e 

 occupied by the stomach. Corals usually live in colonies, or as one com- 

 pound body, attached by a common base to some support, and increase 

 by budding. This physiological relation occasions remarkable grouping: 

 hence the stupendous results in tropical seas, by which the life of the 

 individual is combined with the life of the whole, and the nutriment 

 prepared by each organism is made to contribute to the nourishment of 

 the community of which it forms a part. 



True Coral animals of the sea (the Cyathophylloids chiefly) first 

 appeared in the Trenton Period. The lamelliferous or stony Corals 

 were perhaps most widely diffused and individually abundant in the 

 Silurian age. There is a Palaeozoic and a Neozoic type of Coral. In 

 the former there is a quadripartite arrangement of the plates ; in the 

 latter, the number is some multiple of six. The Secondary Corals more 

 resemble living species of the tropics. Of the 1265 fossil Corals enu- 

 merated by D'Orbigny, 1001 are Neozoic. The outer form of the Cor- 

 ralum, or coral-stock, varies between very great extremes (of lengthen- 

 ing, shortening, flatness, sphericity, &c.) in different individuals of the 

 same genus and species. This fact, and the impossibility of well preserving 

 the delicate, characterizing plates of their inner portions, displayed upon 

 the upper surface, makes it difficult to give any satisfactory series of 

 casts of them. 



